I have no idea. If I had to guess, I'd say that somebody in the film dubs the group the "inglorious bastards", but then when someone decides to write it on the side of their truck, they mess it up. Due to a lack of education, I would think.

I like the spelling, for some reason. It could easily be a piece of the dreaded Whimsy, or Quirkiness, but it seems instead to be one of those odd features that invests a creative project with a kind of integrity, before you even know the first thing about it. Not sure why that is.